A monitoring system for monitoring the pneumatic pressures in pneumatic tires, such as disclosed in US 2002/0047779A1 (JP-A-2002-67638), incorporates in tires of a vehicle detectors which include a pneumatic pressure sensor and a signal transmission circuit for wirelessly transmitting the result detected by the pneumatic pressure sensor. The monitoring system receives electromagnetic wave signals transmitted from the detectors through receivers mounted on the vehicle, so that a monitoring device monitors the pneumatic pressures in the tires based on the receive signal.
In this monitoring system, the receivers that form pairs with the detectors in the tires are mounted near the tires, and the receive data (pneumatic pressure data, etc.) which the receivers have obtained from the detectors are transmitted to the monitoring device.
In this monitoring system, if the receive data are directly transmitted to the monitoring device at a moment when the receiver has received electromagnetic wave signals transmitted from the detectors of other vehicles or from the detectors of other tires that are not corresponding to the receiver, then, the monitoring device may erroneously determine a decrease in the pneumatic pressure.
Therefore, it is proposed in U.S. Pat. No. 5,602,524 (JP-7-507513) that a monitoring system imparts an identification data specific to a tire to a transmission signal at the time when a detector transmits a detected result. Thus, a receiver determines, based on the identification data, whether the receive data be transmitted to a monitoring device.
In the above conventional monitoring systems, however, the receivers are connected to the monitoring device through dedicated signal lines, and only a predetermined number of receivers are connected to the monitoring device. Therefore, different systems must be built up for each of the vehicles on which the receivers of different numbers are installed. Namely, the receivers and the monitoring devices cannot be commonly used for the vehicles of different kinds.
In the case of large vehicles such as trucks and buses, the number of tires and the arrangement thereof are different depending upon the kind of the vehicles. To build up a monitoring system for monitoring the pneumatic pressures in the tires of large vehicles, therefore, the monitoring system must be built up for each of the types of the vehicles by using the receivers and the monitoring devices that meet the kind of the vehicles. Therefore, it is not possible to use the receivers and the monitoring devices in common for the vehicles of different kinds.
In order to solve the above problem, another monitoring system can be contrived in which a plurality of receivers necessary for building the monitoring system are connected to the monitoring device through a communication line permitting the monitoring device to obtain receive signals from the receivers through the single communication line, the detectors incorporated in the tires transmit identification data specific to the tires in addition to the detected results of pneumatic pressures as in U.S. Pat. No. 5,602,524, and the receivers transmit to the monitoring device through the single communication line the signals received from the detectors.
Namely, the monitoring device obtains receive signals from a plurality of receivers through a single communication line, and monitors the pneumatic pressures in the tires based upon the identification data of the tires included in the receive data. Thus, it is possible to monitor the pneumatic pressures in all tires of the vehicle despite of a change in the number of the receivers.
In order for the monitoring device to obtain receive data from the plurality of receivers through a single communication line, however, the receivers must transmit the receive data in response to an instruction from the monitoring device. For this purpose, identification data for communication must be imparted to the receivers, and the monitoring device must send instructions to the receivers based on the identification data.
In this case, identification data must be set for each of the receivers that are installed. Besides, the identification data that are set must be registered to the monitoring device. After all, among the vehicles of different types, it is not possible to build a monitoring system for monitoring pneumatic pressures in vehicle tires by using the same receivers or the monitoring device.